A model was developed to calculate the response of films to heavy ion irradiation as a function of particle type and energy. It is based on the local effect model and was extended to be applied to the non-homogeneous target of photographic emulsions. The calculation was performed for protons and carbon ions in the energy range from 2 to 430 MeV u(-1). The calculated film responses are in very good agreement with experimental data. Based on this model calculation a data base of film responses was set up. This enables a three-dimensional dose verification with films in the heavy ion tumour therapy project at GSI, Darmstadt.